This invention relates to apparatus for the infusion of medical liquids into patients and is particularly concerned with infusion sets and pressure monitors which are used with infusion pumps. Frequently it is required that particular medical liquids be infused into the body of a patient at a predetermined rate. It is possible for the tube of an infusion set leading to the patient to be kinked, or get blocked, and, under these circumstances, firstly the medical liquid is no longer infused into the patient which is the first source of danger but, perhaps more importantly, if the tube is subsequently unkinked or the blockage suddenly released a large quantity of the medical liquid is suddenly infused into the patient as a result of the build up of pressure in the kinked or blocked tube. Such a sudden surge of medical liquid into the patient can be life threatening, particularly in the case of some drugs.
It is therefore desirable to provide some form of automatic infusion pressure monitor which monitors the pressure of the medical fluid in an infusion set or tube leading to the patient. Previously such infusion pressure monitoring devices have included a pressure sensor and means to compare the sensed pressure with a preset value corresponding to the maximum allowed delivery pressure. The delivery pressure for an infusion varies with a number of factors such as the viscosity of the medical fluid, the cross sectional area of the needle through which the infusion is performed, the speed at which the infusion of medical fluid is to be introduced into the patient and whether a filter is included in the infusion line. Conventional infusion pressure monitoring devices which include a preset maximum pressure limit typically have their maximum pressure limit set to a value higher than is likely to be encountered with, for example, a high viscosity medical liquid being pumped at a high rate through a filter and fine needle. When such a device is subsequently used with less viscous medical liquids at lower pumping rates through coarser needles the monitoring device is relatively insensitive and so only responds if the infusion tube is completely blocked for a considerable period of time.
Infusion pressure monitoring devices are also available in which an operator sets manually the required maximum delivery pressure. Naturally such devices are subject to human error and, even more importantly, there is a tendency for the operator to set the maximum infusion pressure to a high value so that the operator is not troubled by "false alarms".